In this process, to ensure pumping which extends, in the course of a depressurization from atmospheric pressure, or slightly less if an equilibrium is assured between an adsorber at the beginning of repressurization, to a pressure as low as about 0.2.times.10.sup.5 Pa absolute, with rejection of a residue to the atmosphere, one utilizes generally volumetric compressors of the "Roots" type, which, if they are suitable to handle high flow rates, accept, at these pressure levels, only relatively small pressure differences between the inlet and the outlet, of the order of 0.5.times.10.sup.5 Pa. For higher compression loads, the output of the compressor falls, because one must use two-stage volumetric compressors, the second stage being operated in series with the first stage when the pressure has fallen thanks to the first single stage, to a value of the order of 0.5.times.10.sup.5 Pa absolute, and one sometimes uses, when the pumped flow rate is great, a multi-stage compressor.
This procedure is therefore burdensome particularly as to energy consumption and has difficulties of embodiment for production units with high flow rates.